The present invention relates to an image-forming optical apparatus and, more particularly, to an image-forming optical apparatus which is most suitable for an image pickup apparatus designed to form an image relatively small in size and uses an objective optical system in which at least one reflecting surface having an image-forming power required for image formation is decentered.
There has heretofore been known a compact reflecting decentered optical system as disclosed in Japanese Patent Application Unexamined Publication Number hereinafter referred to as "JP(A)"! 59-84201. This is an invention of a one-dimensional light-receiving lens comprising a cylindrical reflecting surface; therefore, two-dimensional imaging cannot be effected with this conventional optical system. JP(A) 62-144127 discloses an optical system wherein the identical cylindrical surface is used twice to effect reflection in order to reduce spherical aberration in the above-mentioned invention.
JP(A) 62-205547 discloses the use of an aspherical reflecting surface as a reflecting surface, but makes no mention of the configuration of the reflecting surface.
U.S. Pat. Nos. 3,810,221 and 3,836,931 both disclose an example in which a rotationally symmetric aspherical mirror and a lens system having a surface which has only one plane of symmetry are used to constitute a finder optical system of a reflex camera. In this example, however, the surface having only one plane of symmetry is utilized for the purpose of correcting the tilt of a virtual image for observation.
JP(A) 1-257834 (U.S. Pat. No. 5,274,406) discloses an example in which a surface having only one plane of symmetry is used for a reflecting mirror to correct image distortion in a rear projection type television. In this example, however, a projection lens system is used for projection onto a screen, and the surface having only one plane of symmetry is used for correction of image distortion. An example of a back-coated mirror type decentered optical system using an anamorphic surface and a toric surface as an observation optical system is also disclosed. However, the decentered optical system is not sufficiently corrected for aberrations, including image distortion.
None of the above-described prior arts use a surface having only one plane of symmetry as a back-coated mirror to form a folded optical path.
JP(A) 8-292368, 8-292371 and 8-292372 each disclose an image pickup optical system (i.e. a fixed focal length optical system or a zoom optical system) using a surface having only one plane of symmetry as a reflecting surface. However, the disclosed image pickup optical system has an unfavorably long optical path length from an entrance surface of an optical system constituent element including a rotationally asymmetric surface to an exit surface thereof or from a rotationally asymmetric surface of the optical system that is closest to the object to a rotationally asymmetric surface thereof that is closest to the image (in an example, image formation takes place once in the course of travel of light along the optical path). This causes the optical system to increase in size. Therefore, there is no merit in using rotationally asymmetric surfaces, which are difficult to produce.
Incidentally, to remove moire fringes appearing in an image pickup apparatus due to superposition of the repeating period of the pixels of an image pickup device and a spatial frequency component in an object image which is close to the repeating period, JP(A) 7-325269 proposes a low-pass filter which enables such moire patterns to be effectively removed by using a double image formed by pupil division and which is less costly and effective even under defocus conditions.
In the conventional rotationally symmetric optical systems, a transmitting rotationally symmetric lens having a refracting power is assigned to exert the required refracting power. Therefore, many constituent elements are needed for aberration correction. In the conventional decentered optical systems, an imaged figure or the like is undesirably distorted and the correct shape cannot be recorded unless the formed image is favorably corrected for aberrations, particularly rotationally asymmetric distortion.
In a rotationally symmetric optical system comprising a refracting lens which is formed from a surface rotationally symmetric about an optical axis, a straight-line optical path is formed. Therefore, the entire optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large-sized image pickup apparatus.